1. Field of the Invention
The present invention relates to methods for inputting pulse signals to the gates of FETs (field-effect transistors) and measuring the IV (current-voltage) characteristics of the FETs. More specifically, the present invention relates to a method preferably used for measuring the IV characteristics of advanced FETs, such as MOSFETs (metal oxide semiconductor field-effect transistors) manufactured by SOI (silicon-on-insulator) technology or strained-silicon fabrication technology.
2. Description of the Related Art
The IV characteristic of an FET is determined by measuring a drain current flowing when a predetermined DC (direct current) voltage is applied to the gate of the FET, while a predetermined bias voltage is applied to the drain of the FET.
However, when the IV characteristics of an advanced FET, such as a MOSFET, manufactured by SOI technology or strained-silicon fabrication technology are measured, the known measuring scheme in which a DC voltage is applied to the gate of the FET (hereinafter referred to as a “device under test (DUT)”) may not provide reliable measurement results of the IV characteristic, because of the self-heating phenomenon exhibited by the DUT.
Accordingly, a measuring method in which a short-duration pulse is applied to the gate of the DUT has been proposed, for example, as described in K. A. Jenkins and J. Y-C. Sun “IEEE Electron Device Letters”, Vol. 16, No. 4, April 1995, p135 to 147. This method allows the semiconductor device to be operated without producing heat, thus making it possible to provide a measurement result that is not affected by heat.
In the measuring method in which the pulse is applied to the gate of the DUT, a predetermined voltage is applied to the drain of the DUT via the so-called “bias tee”, so that drain current to be measured flows while the pulse is applied.
In this case, the impedance of the bias tee and the measurement unit as viewed from the DUT (i.e., the input impedance of the measurement unit) serves as a load on the DUT. Thus, when the drain current flows to the DUT, the drain voltage drops by an amount corresponding to a voltage drop caused by the load. This makes it impossible to measure the drain current under a predetermined drain voltage, which is a major cause of the measurement error of the IV characteristic. The drop in the drain voltage also restricts the range of drain voltages, and disadvantageously the IV characteristics in the necessary drain voltage range cannot be measured.